Superconductivity was first observed by the Dutch physicist H. K. Onnes in 1911 during his investigations of the electrical conductivities of metals at very low temperatures. He observed that as purified mercury is cooled, its electrical resistivity vanishes abruptly at a temperature of 4.16K. Above this temperature, the electrical resistivity is small but finite and measurable; alternatively, when the temperature is reduced below 4.16K., the electrical resistivity is so small that it is effectively zero. This distinct temperature at which the transition and loss of effective electrical resistivity occurs has been termed the critical temperature or "T.sub.c ". Onnes believed he had discovered a new physical state of matter at temperatures below the critical temperature and coined the term "superconducting state" for the observed phenomenon at temperatures below the critical temperature (T.sub.c) and the term "normal state" for the electrical properties observed at temperatures above the critical temperature. Onnes also found that the superconducting transition is reversible and that the superconducting material recovered its normal, electrical resistivity at the critical temperature.
The modern theory of superconductivity is the result of the research investigations by Bardeen, Cooper, and Schrieffer [Phys. Rev. 106:162 (1957)]. Their proposal, conventionally known as the "BCS theory", has now gained universal acceptance because it has proved capable of explaining most of the observed phenomena relating to superconductivity. Their principles employ a quantum mechanical treatment of the superconductive phenomenon; and their theory has been employed to explain the various observable effects such as zero electrical resistance, the Meissner effect, and the like. Since the BCS theory is so steeped in quantum mechanics, the reader is directed to published texts in the scientific literature for a complete description and explanation. These include: M. A. Omar, Elementary Solid State Physics: Principles and Applications, Addison-Wesley Publishing Company, 1975, pages 496-527; M. Tinkham, Introduction to Superconductivity, McGraw-Hill Co., 1975.
Superconductivity has been found not to be a rare phenomenon. It is exhibited by a substantial number of atomic elements, metallic alloys, and most recently, refractory oxide ceramics. For many years, the highest known critical temperature was only 23K. There has, accordingly, been intense interest and research investigations into finding superconductive materials with much higher critical temperatures, most desirably those which hopefully would approach room temperature (20C.). Until relatively recently, efforts to achieve this goal have met with complete failure. Beginning about 1986, however, polycrystalline sintered ceramic pellets of yttrium-barium-copper oxide and mixtures of potassium, barium, bismuth, and oxygen without copper have been found to demonstrate relatively high critical temperatures (T.sub.c) and superconductivity at temperatures up to 120K. [Bednorz, J. G. and K. A. Muller, Z. Phys. B64:189 (1986); Wu et al., Phys. Rev. Lett. 58:905 (1987); and Chu et al., Phys. Rev. Lett. 60:941 (1988)]. These compounds are now conventionally termed "high T.sub.c " superconductors.
Since about 1986, however, the interest in superconductive materials as potential replacements for conventionally known metal alloy wiring and microcircuitry has risen appreciably; and the search for ever-higher T.sub.c superconductors in various formats is presently an area of intense exploration. Merely representative of these continuing research investigations and recently reported developments are the following publications: Experimental Techniques in Condensed Matter Physics at Low Temperatures, (R. C. Richardson and E. N. Smith, editors), Addison Wesley Inc., 1988, pages 118-123; G. K. White, Experimental Techniques in Low-Temperature Physics, Oxford University Press, 1959, pages 295-298; Advances in Superconductivity, Proceedings of the 1st International Symposium on Superconductivity, August 1988, Nagoya, Japan; Yeh et al., Phys. Rev. B36:2414 (1987); Morelli et al., Phys. Rev. B36:3917 (1987); Chaudhari et al., Phys. Rev. B36:8903 (1987); Tachikawa et al., Proc. IEEE 77:1124 (1989); Tabuchi et al., Appl. Phys. Lett. 53:606 (19898); Sacchi et al., Appl. Phys. Lett. 53:1111 (1988); Abell et al., Physica C162-164:1265 (1989); Bailey et al., Physica C167:133 (1990); Xiao et al., Phys. Rev. B36:2382 (1987); Matsuda et al., Mat. Res. Soc. Symp. Proc. 99:695 (1988); Witanachchi et al., J. Mater. Res. 5:717 (1990); Superconductive Industry, Winter, 1989, page 31; Engineer's Guide to High-Temperature Superconductivity, Wiley & Sons, Inc., 1989; and D. Newman, Superconductive Industry 3:16 (1990).
A concomitant and continuing problem has also arisen regarding the electrical joining of superconductive materials, particularly the juncture of high T.sub.c superconductors to each other and to other electrically conductive materials in the normal state at temperatures between 70K. and 300K. and to conventional superconductive materials which have a transition temperature below 20K. By definition, electrically conductive materials in the normal state include both the normal conductors such as gold, silver, copper, and iron; and the semi-conductors such as carbon, silicon, gray tin, and germanium as well as their mixtures with indium, gallium, antimony, and arsenic. It is also difficult to make effective adhesive and low resistance junctions to the atomic elements and alloys most frequently use in practical superconducting applications. These are the conventional superconductors Nb, NbTi, and NbSn; and they typically serve as materials used to make superconducting motors, generators, and magnets which operate at liquid helium temperature (4.5K.).
Traditionally, solders--a general term for alloys useful for joining metals together by the process of soldering--have been used to electrically join conductors to themselves and to semi-conductors. The principal types of solder conventionally known are: soft solders such as lead-tin alloys; and brazing solders such as alloys of copper and zinc and sometimes silver. Representative of conventionally known solders and soldering techniques are U.S. Pat. No. 3,600,144 describing a low melting point brazing alloy; U.S. Pat. No. 4,050,956 describing a method of chemically bonding metals to refractory oxide ceramics; U.S. Pat. No. 4,580,714 disclosing a hard solder alloy comprising copper, titanium, aluminum, and vanadium; U.S. Pat. No. 4,582,240 revealing a method for intermetallic diffusion bonding of piezo-electric components; U.S. Pat. No. 4,621,761 identifying a brazing process for forming strong joints between metals and ceramics while limiting the brazing temperature to not more than 750C.; and U.S. Pat. No. 4,631,099 describing a method for adhesion of oxide type ceramics and copper or a copper alloy.
More recent attempts to refine specialized techniques for lowering the resistance of electrical contacts between two superconductive materials include vapor deposition of silver followed by annealing bulk sintered samples of yttrium-barium-copper oxide at temperatures up to 500C. for an hour [Superconductor News, May-June, 1988, page 5]; the use of laser energy ot deposit a thin film of superconductive yttrium-barium-copper oxide direclty onto a silicon substrate [Superconductor News, May-June, 1988, page 1]. Sputter depositing a layer of silver on a yttrium-barium-copper oxide surface [Elkin et al., Appl. Phys. Lett. 52 (1988)]; deposition of silver or gold on a superconductive material [Van der Mass et al., Nature 328:603 (1987)]; and thermal evaporation of silver on a yttrium-barium-copper oxide surface [Tzeng et al., Appl. Phys. Lett. 52 (1988)]. The problem with all these methods is that each requires the extensive use of vacuum deposition equipment.
Clearly, therefore, there remains a well recognized need for tangible means by which to join normal conductors and semi-conductors which are electrically functional at temperatures between 300K. to 70K. to those low temperature superconductors which become electrically valuable below 10K. If such an electrically conductive article were available, the electrical junction and union between the electrical circuits and instruments of our everyday world could then be linked and employed in combination with the minimal electrical resistance of circuits provided by superconductors generally. Such a linkage and means for electrical juncture, however, have previously been unknown in this art.